URINARY ORGANS 



461 



ftt 



penetrated by deep furrows and clefts into which the veins 

 extend ; posteriorly they may fuse together in the middle line, 

 as in Lizards. 



There is not always a perfect symmetry between the organ 

 of either side, and this is most marked in Snakes, in which the 

 greatly tabulated kidneys, like those of limbless Lizards, are 

 elongated, narrow, and band-like, in correspondence with the form 

 of the body. 



A urinary bladder, arising from the ventral wall of the cloaca, is 

 present in most Lizards and Chelonians ; it is more or less bilobed. 

 A bladder is wanting in Snakes, Crocodiles, and Birds, as well as in 

 Monitors and Amphisbsenians amongst Lizards. It is derived in 

 part from the stalk of the allantois and in part from the cloaca l 

 (cf. p. 440). 



Mammals. The definitive kidneys of Mammals 2 are propor- 

 tionately small, and lie on the quadratus lumborum muscle and 

 ribs. They usually possess a convex 

 outer, and a concave inner border ; 

 the latter is called the hilum, and 

 at this point the ureters arise and 

 the blood-vessels enter. The ex- 

 panded proximal portion of the ureter 

 is divided up to form one or more 

 calyces into which small papilliform 

 processes of the pyramids project 

 (Fig. 347) ; on the summits of these 

 the urinary tubules open in varying 

 number. The calyces are continuous 

 with a large cavity in the widened 

 portion of the ureter called the pelvis. 

 and from this the ureter (metanephric @ a ca iy ces ; M, 

 duct) passes freely backwards for some 

 distance to open into the bladder on 

 its dorsal side, sometimes nearer the 

 apex, sometimes towards the fundus. 

 The bladder communicates with the 

 urino genital canal or urethra (cf. under 

 Genital organs). 



The kidney is greatly tabulated in the embryo ; this condition 

 may remain throughout life (e.g. in Cetacea, Pinnipedia, Probos- 

 cidea, and certain Ungulates, Carnivores, and Primates, cf. Fig. 

 348), or the lobes may become more or less completely united. In 

 the latter case the original division into lobes may still be recog- 

 nised to a greater or less extent internally. A section of the 



1 The urinary bladder is said to be represented in embryo Birds by an 

 enlargement of the stalk of the allantois, and in Crocodiles by a ventral outgrowth 

 from the cloaca. 



2 In the embryonic mesonephros, nephrostomes occur only in Echidna. 



:G. 347. DIAGRAMMATIC LONGI- 

 TUDINAL SECTION THROUGH THE 

 KIDNEY OF A MAMMAL. 



, medullary 

 substance arranged in pyramids 

 (Pr) ; between the latter the 

 cortical substance extends in 

 the form of the columns of 

 Bertini (B, B] ; R, R, cortical 

 substance ; Pe, pelvis ; Ur, 

 ureter. 



